There are many ways to make sure that their is no cheating for the trades. You can assign one GM to look over all the trades to make sure there fair. Like one of the guys said on this forum that when you make a trade it should be up for 3-6 days so itt can be accepted or vetoed so there would not be any cheating. There are many ways you can make sure there is no cheating.
one big problem is that this would be on the hand a lot of work, and adjusting prices is for the most gm the toughsest job to do, since nearly none BB player looks exactly like the other and you need the exact price for both of them checking the trade.
But it will also lead to a lot of spam mails(which gm had to handle also), since skills ain't known and it is hard to figure them out from looking at the stats so many people would just ask, and also when you scout on the transferlist not everybody would be pleased to get mails like "i saw you are selling X, i have Y or Z for a trade did you like them." especially when the price is like that he most likely get sold anyway ;)
Also the gain for the gain would be minimal, since when you trade is it pretty unlikely that both player get the exactly player they like, cause i believe it quite unlikely that player X has a player player Y desires and it works also the different way.* So most managers would be better off, when the interest sells his player to get the money to buy the player he likes and them he could use this money to buy the player he wants to have.
* with more team you have the more unlikely is it, and it is not really easy in the NBA with just 30(?) teams.